Why is this Sunday called “Septuagesima”?
Because in accordance with the words of the First Council of Orleans, some pious Christian congregations in the earliest ages of the church, especially the clergy, began to fast seventy days before Easter, on this Sunday, which was therefore called “Septuagesima” – the seventieth day. The same is the case with the Sundays following, which are called Sexagesima, Quinquagesima, Quadragesima, because some Christians commenced to fast sixty days, others fifty, others forty days before Easter, until finally, to make it properly uniform, Popes Gregory and Gelasius arranged that all Christians should fast forty days before Easter, commencing with Ash Wednesday.
Why, from this day until Easter, does the Church omit in her service all joyful canticles, alleluias, and the Gloria in excelsis, etc.?
Gradually to prepare the minds of the faithful for the serious time of penance and sorrow; to remind the sinner of the grievousness of his errors, and to exhort him to penance. So the priest appears at the altar in violet, the color of penance, and the front of the altar is covered with a violet curtain. To arouse our sorrow for our sins, and show the need of repentance, the Church in the name of all mankind at the Introit cries with David:
The groans of death surrounded me, the sorrows of hell encompassed me: and in my affliction I called upon the Lord, and he heard my voice from his holy temple (Ps. 17:5-7). I will love thee O Lord, my strength; the Lord is my firmament, and my refuge, and my deliverer. (Ps. 17:2-3). Glory be to the Father.
Taken from Fr. Leonard Goffine’s The Church’s Year.